This invention relates to a method of producing a porous body for a solid electrolytic capacitor, and more particularly, to a method of producing titanium-aluminum a (Ti-Al) alloy porous body.
At present, tantalum (Ta) has been widely used as a valve metal for a solid electrolytic capacitor. Notwithstanding its excellent characteristics, however, the future of the Ta capacitor is not very bright because there is a shortage of Ta resources. Since a large amount of Ta has been consumed as a capacitor material in recent years, the Ta ore, which is originally rare, has become nore and more rare, and its price has soared as supplies dwindle. It is expected that sooner or later, the Ta capacitor must lose it marketability due to its soaring price. Accordingly, a development of a solid electrolytic capacitor, made of other materials, is urgently needed to replace the Ta capacitor.
As a valve metal for a solid electrolytic capacitor that will replace Ta, a Ti-Al alloy has been described. One of these description is includes in U.S. Pat. No. 3,599,053 entitled "IMPROVED TITANIUM ALLOY CAPACITOR". This patent relates to an electrode for an electrolytic capacitor using a Ti-Al or Ti-Al-Zr alloy porous body, and discloses capacitor characteristics of a dielectric layer formed by anodically oxidizing the surface of either the Al-Ti or Al-Ti-Zr alloy. In this prior art, the Ti-Al or the Ti-Al-Zr alloy porous body is formed by using Ti, Al and Zr powder as the starting materials. Then, these alloy powders are pressed into a predetermined shape. Thereafter, the pressed body is sintered at a temperature which is higher than the melting point of aluminum.
However, the present inventors believe that the sintered body produced from the starting Al-Ti alloy powder can not be used as an anode for an electrolytic capacitor that will replace Ta. When the Al-Ti alloy is a fine-grained powder, the amount of surface oxidation increases. That is, if a porous body comparable to Ta is to be formed, powder of a gran size in the order of about several microns must be dealt with. If a porous body is produced by using such micro-fine powder, however, large quantities of surface oxides remain. The result is a remarkable degradation of the capacitor characteristics.
Incidentally, the satisfactory capacitor characteristics can be maintained if the porous body is produced by using a micro-fine powder of Ta alone, primarily for the following reason. Besides the fact that Ta is originally a metal which has a low degree of natural oxidation, the sintering of the Ta powder is effected at a temperature which is higher than the decomposition point of the Ta oxides. In conjunction with this point, it is to be noted that Al or Ti or their alloys form oxides which are more stable than Ta, and these oxides can not be virtually decomposed by the heat-treatment in vacuum.
Meanwhile, the present inventors previously described in U.S. patent application Ser. No. 82,217 how to obtain a porous body of a Ti-Al alloy by first compression-molding the Ti powder and Al powder. Then the resulting molded article is heat treated at a temperature which is higher than 500.degree. C., but below the melting point of Al. According to this prior application, it is possible to obtain a porous body having a specific surface area which is greater than the surface area of the aforementioned Ti-Al alloy porous body, and hence, to obtain a capacitor having greater capacitance. However, this prior patent application is not free from the drawback that since the micro-fine Ti and Al powder are employed as the starting materials, a natural oxidation of the Ti and Al powder is unavoidable during the production process.
As described above, these heretofore known methods of producing the Ti-Al alloy porous body fail to provide a product having a large specific surface area. Even if the product having a desired specific surface area can be obtained, the product unavoidably possesses inferior capacitor characteristics due to the surface oxidation of the Ti powdery particles. More particularly, its leakage current and dielectric loss characteristic become inferior. To prevent oxidation of the micro-fine powder during the production process, strict management of the starting powder is necessary but this in turn results in an increase in the production cost.